1. Field of the Invention
This invention relates to electronic heart pacing apparatus and in particular to cautery protection circuits associated therewith.
2. State of The Prior Art
Heart pacers such as that described in U.S. Pat. No. 3,057,356 issued in the name of Wilson Greatbatch and assigned to the assignee of this invention, are known for providing electrical stimulus to the heart whereby it is contracted at a desired rate in the order of 72 beats per minute. Such a heart pacemaker is capable of being implanted within the human body and operative in such an environment for long periods of time. Typically, such pacemakers are implanted in the pectorial region or in the abdominal region of the patient by a surgical procedure, whereby an incision is made in such region and the pacemaker with its own internal power supply, is inserted within the patient's body.
In FIG. 1, there is shown an output portion of an electronic heart pacemaker of the prior art. The output circuitry includes a transistor Q'12 which is periodically turned on and off at a rate corresponding to that with which the patient's heart is to be stimulated, e.g. 72 beats per minute, and for a duration sufficient to stimulate the patient's heart. The base and collector electrodes are connected respectively to suitable biasing resistor R'19, and capacitor C'8 and charging resistor R'20, and the output is coupled from the collector of the transistor Q'12 by a suitable capacitor C'8. A zener diode CR'10 is connected across the output of the circuit to provide defibrillation protection. In the normal functioning of the heart, an electrical charge is established across the muscle tissue of the heart, i.e. polarization, and is subsequently discharged, i.e., depolarization. In fibrillation, there are many origins of depolarization which interact with each other and, as a result, the heart assumes a random motion, whereby little if any blood is circulated in the arterial system of the patient. To reinitiate the normal activity of the heart, a defibrillation pulse of relatively large amplitude is applied across the patient's heart. Typically, a pair of paddles (electrodes) is placed on each side of the patient's chest, whereby the defibrillation pulse is applied to his heart to reinitiate the normal rhythmic operation of his heart. The defibrillation pulse as seen by the output portion of the heart pacemaker circuit is in the order of 1500V. It is expected that such a large voltage could easily damage if not destroy the circuit elements of the circuit unless otherwise protected. To prevent this, the zener diode CR'10 is inserted across the output, thereby limiting the voltage applied to the pacemaker circuitry to a safe level, e.g. 8V.
The surgical procedure for implanting or removing the heart pacemaker into the body of the patient may involve cauterizing the incision made for the pacemaker pocket, thereby sealing off the small blood vessels surrounding the pocket. In FIG. 2, there is shown a patient with a heart pacemaker 10 implanted therein and the use of a cautery electrode 12 for cauterizing the pacemaker incision. Typically, a cautery unit such as the Bovie Electrosurgical Unit applies an electrical signal such as shown in FIG. 3A to the electrode 12. The high frequency signal has a "damped" waveform; the term damped means that the current is in pulses which start with a maximum amplitude and decrease in amplitude at a logrithmic rate. These groups of pulses are sometimes referrred to as wave trains and the number of these wave trains occurring per second is called the wave train frequency. The rate at which the pulses occur in each wave train (the number per second) denotes the frequency of the unit, e.g. 500 to 800 kilocycles per second. In the following table, I show values for several characteristics of the two basic currents. The values are approximate, but at the same time representative of current practice.
__________________________________________________________________________ OSCILLATING WAVE TRAIN PEAK OUTPUT MAXIMUM CURRENT FREQUENCY FREQUENCY VOLTAGE (NO LOAD) OUTPUT __________________________________________________________________________ Cutting 500-800 KHz 30000-50000/sec 3000-3500 volts 250 watts Coagulating 500-800 KHz 10000-15000/sec 5000-7500 volts 150 watts __________________________________________________________________________
As shown in FIG. 2, an electric field 18 is established between the cautery electrode or forcep 12 and a cautery ground plate 16 disposed against the patient's buttocks. As shown in FIG. 2, the artificial heart pulse generator 10 and its electrode 14 are disposed in the path of the field 18, whereby a signal is readily induced into the output portion of the heart pacemaker circuitry.
Through experimentation with canines, it is known that serious problems occur, not necessarily if such cautery electric field inhibits the pulse generator for a brief moment, but rather if certain extraneous signals are induced into the output portion of the heart pulse generator circuitry. In a further distinct effort, I have discovered in the course of experimentation that if an unsymmetrical waveform, as shown in FIG. 3B, is induced into the output portion of the heart pacemaker circuitry, as shown in FIG. 1, and applied by the pacemaker electrodes to the heart, the heart may be induced into fibrillation.
As can be understood with regard to FIG. 1, the signal induced into the output section of the heart pacemaker circuitry by the intense field established by cautery, is rectified by the diode CR'10 to produce the unsymmetrical wave as shown in FIG. 3B. My tests have revealed that such unsymmetrical waveforms are particularly effective, as compared with symmetrical waveforms, to stimulate the heart and thereby induce fibrillation into the heart, whereby the normal, rhythmic polarization and depolarization of the heart is interrrupted and it begins to vibrate in a relatively uncontrolled fashion.
In the normal operation of the heart, a negative charge is established upon the exterior wall of the heart muscle cells, whereas a positive charge is established therein. Then, as a spontaneous depolarization occurs, whereby the positive and negative charges appear to move toward each other, the heart cells quickly contract, the polarization dissipates, and the cells expand and repolarize more slowly. Coordination of the rhythmic polarization and depolarization of the heart muscle cells is effected by the heart's own pacemaker cells and the heart functions of rhythmically pump blood throughout the arterial system. Though the mechanism for inducing the heart into fibrillation is not completely understood, it is possible that a coincidence between the noted unsymmetrical wave and the repolarization of the heart muscle cells may be particularly effective to induce the heart into fibrillation.
The cautery procedures described above are particularly prone to induce unsymmetrical signals upon the output circuit of unipolar-type heart pacemakers. In particular, such pacemakers include a first or stimulator electrode disposed through a vein into the patient's ventricle, and a second or indifferent electrode disposed adjacent to the pacemaker. During the implantation or removal procedures, the cautery forcep may be applied to the incision leading to the pacemaker pocket within the patient. Thus, due to the proximity of the cautery electrode and the pacemaker housing, and in particular the indifferent electrode, the amplitude of the unsymmetrical signal appearing in the output portion of the heart pacemaker circuitry is particularly high. Thus, in cauterizing the incision after the heart pacemaker has been installed or before it is removed, the cautery forceps are brought very close, if not in contact with the housing of the pacemaker, thus presenting a very real problem in the induction of the undesired, unsymmetrical signal in its output circuitry.
In U.S. Pat. No. 3,757,791, there is disclosed an artificial heart pacemaker incorporating distinct atrial and ventricular pulsing circuits synchronized with each other. In the output secton of each such pulsing circuit, there is incorporated a pair of zener diodes coupled in series to each other in opposing fashion to safeguard their respective circuits from excessive signals appearing across the output electrodes from an external source. In particular, there is disclosed that if defibrillation equipment is used, very high voltage may be applied to the patient's heart and that such zener diodes are incorporated to protect the pacemaker circuitry and to short-circuit the large voltage signals therethrough. The noted patent does not disclose any relationship between the described defibrillation protection circuit and the problems occurring during cautery procedures, whereby unsymmetrical signals may be applied to the heart and, in particular, there is no teaching that the defibrillation protection measures are related to shaping the cautery-induced signals.